An installation for transmission of high-voltage direct current between two alternating-voltage networks comprises two converter stations, each connected on its a.c. side to a separate one of the alternating-voltage networks, and a common d.c. connection. The d.c. connection may be in the form of an overhead line and/or cable and may also in certain parts consist of ground or water instead of a metallic conductor. Each of the converter stations comprises a converter, series capacitors, usually at least one converter transformer, as well as shunt filters for generation of reactive power and filtering of harmonics. The converters are normally line-commutated, current-source converters, by which it is to be understood that the current commutation between the valves of the converters takes place by means of voltages occurring in the alternating-voltage network, and that the d.c. connection, viewed from the converters, occurs as a stiff current source.
For the purpose of reducing the harmonics generated by the converters, especially the 5th and 7th harmonics, each of the converters usually consists of two mutually series-connected six-pulse bridges, the converter transformer being provided with two secondary windings with a mutual phase shift of 30.degree.. Each of the converter bridges is connected to the alternating-voltage network via series capacitors and a separate secondary winding on the converter transformer.
During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other, hereinafter referred to as the inverter, operates in inverter operation. Control equipment for the respective converter generates a control signal corresponding to a control angle .alpha. at which firing pulses are applied to the valves of the converters.
For the purpose of minimizing the consumption of reactive power by the converters, and reducing the stresses on components included in the converter stations, it is advantageous to control the rectifier with the smallest possible control angle .alpha. and to control the inverter with a control angle which results in the smallest possible extinction angle .gamma. (margin of commutation) without jeopardizing the controlled operation. The control system of the installation is, therefore, usually designed such that the inverter is controlled to a suitable maximum direct voltage for the operating conditions of the installation, taking into consideration safety margins with respect to commutating errors, voltage variations on the a.c. network, and other deviations from nominal operation which may occur whereas the rectifier is controlled by current control. The reference value of the current control is formed in dependence on a current order, which in turn is formed in dependence on a power order and the prevailing direct voltage in such a way that the direct current and hence the transferred active power remain at a desired value.
Usually, the control equipment for rectifiers and inverters is designed identically, whereby in the rectifier a current controller is activated and in the inverter control equipment for a control with the aim of maintaining the extinction angle at, but not lower than, a preselected lowest value is activated. Between the control angle .alpha., the extinction angle .gamma., and the overlap angle u, the known relationship .alpha.+u+.gamma.=180.degree. prevails. The control equipment of the inverter is usually designed such that its control angle is formed in dependence on a limiting signal. Such a limiting signal may, in turn, be formed in dependence on sensed values of direct current and available commutating voltage, in dependence on predicted values of the extinction angle, or by means of a feedback control of the extinction angle.
The control system of the converters is usually designed to generate firing pulses to the respective valves with mutually identical intervals, so-called equidistant control.
With series compensation, several advantages are obtained. The series capacitors are charged periodically by the current which flows through them and the voltage across the capacitors thus generated provide an addition to the commutating voltage across the valves of the converter. The commutating voltage becomes phase-shifted relative to the voltages of the alternating-voltage network in such a way that, with control and extinction angles still related to the phase position for the voltages of the alternating-voltage network, the valves in rectifier operation may be controlled with control angles smaller than zero and in inverter operation with extinction angles smaller than zero (although the commutating margin, related to the commutating voltage of the valve, is, of course, greater than zero). This makes possible a reduction of the reactive power consumption for the converters. In this way, the need of generation of reactive power in the shunt filters is reduced, and the shunt filters may thus be dimensioned substantially on the basis of the need of harmonic filtering.
The charging current of the capacitors and hence the voltage thereof are proportional to the direct current in the d.c. connection, and by suitable dimensioning of the capacitors, the dependence of the overlap angle on the magnitude of the direct current may be compensated for. This means that the series compensation contributes to maintaining the commutating margin of the valves also in case of fast current transients. The control of the inverter means that the inverter, at least without series compensation, exhibits a negative current/voltage characteristic, which, especially in those cases where the d.c. connection comprises a long cable, in case of voltage reductions in the alternating-voltage network may lead to an avalanche-like growth of the current. The series compensation influences the current/voltage characteristic of the inverter in a stabilizing direction, and by a suitable choice of series capacitors it may also be brought to be positive.
For a general description of the technique for transmission of high-voltage direct current, reference is made to Erich Uhlmann: Power Transmission by Direct Current, Springer Verlag, Berlin Heidelberg New York 1975.
A general description of the mode of operation of the converter station with series capacitors introduced into the ac connections between the converter transformer and a converter in a six-pulse bridge connection is given in John Reeve, John A. Baron, and G. A. Hanley: A Technical Assessment of Artificial Commutation of HVDC Converters with Series Compensation (IEEE Trans. on Power Apparatus and Systems, Vol. PAS-87, October 1968, pages 1830-1840).
However, series compensation of the converter station means that the commutating voltage of the valves is dependent on both amplitude and phase for the current-dependent voltage across the respective series capacitor. During stationary undisturbed operation with symmetrical phase currents, the mean value of the voltage across the respective capacitor remains equal to zero and the capacitor voltages are identical between themselves. Their contribution to the commutating voltage thus remains equal for all the valves included in the converter bridge. In case of fast changes in direct current and/or control angle for the converter bridges, however, capacitors in different phases will be charged with a different current-time area and hence assume different voltages. An unbalance in capacitor voltage and hence in commutating voltage thus arises between the different phases.
When the operating state returns to stationary undisturbed operation, the unbalance is reduced by itself in that it entails different overlap angles and conduction intervals for different valves. This process, however, occurs relatively slowly, especially during operation with low current and may typically take several seconds.
The unbalance also means that the voltage across the direct-voltage terminals of the converter bridges will contain an alternating-voltage component of the same frequency as the fundamental-tone frequency of the connected alternating-voltage network, usually 50 or 60 Hz. A method and a device for damping oscillations in a power transmission system at or near fundamental frequency are described in the patent specification WO 94/07291.
However, an unbalance in the capacitor voltages also means that the commutating margin is reduced for certain valves in the converter bridge, which in turn entails an increased risk of commutating errors. This increased risk remains, although to a decreasing extent, also during the time the unbalance is reduced.